1. Field of the Invention
The present invention relates to an optical emitting module that emits light of a light-emitting device with a desired light radiation intensity distribution. In particular, the present invention relates to an optical emitting module that, in an optical space transmission system that transmits information data, such as video data, sound data, or other digital data, in the form of a light signal between an optical space transmitter and an optical space receiver through a free space, is used for the optical space transmitter or the like.
2. Description of the Related Art
In a light-emitting device, such as a semiconductor light-emitting device (LED) or a semiconductor laser (LD), a light radiation intensity distribution (a change in radiation intensity with respect to the angle of the principal light axis of a light-emitting device) is determined in accordance with manufacturing or the like of the light-emitting device. However, a preferable light radiation intensity distribution differs in accordance with the purpose of the light-emitting device, and there are many cases where the light radiation intensity distribution of the light-emitting device does not become a light radiation intensity distribution suitable for the purpose. For this reason, it is necessary that an optical emitting module is provided with a lens or the like on the front surface of the light-emitting device such that the light radiation intensity distribution is approximated to the light radiation intensity distribution suitable for the purpose.
For example, in an optical space transmission system that transmits information data in the form of a light signal between an optical space transmitter and an optical space receiver through a free space, there is a purpose of securing a constant transmission distance when the angle from the principal light axis is within a transmission angle range ±α. At this time, if the angle of a light beam emitted from the optical emitting module with respect to the principal light axis of the optical emitting module is an emission angle θ1, a light radiation intensity distribution is preferable such that as little light as possible is emitted at the emission angle θ1 greater than α, and the light radiation intensity is substantially uniform when the emission angle θ1 is in a range of 0 to α.
A known optical emitting module which approximates the light radiation intensity distribution uses a prism (for example, Japanese Patent Unexamined Publication No. 11-14935). FIG. 23 shows a known optical module described in Japanese Patent Unexamined Publication No. 11-14935.
Referring to FIG. 23, optical emitting module 100 is provided with prism 102 on the front surface of light-emitting device 101. Prism 102 has a parallel central portion and an inclined peripheral portion. Out of light beams emitted from the center of light-emitting device 101, light beams with the emission angle θ0 equal to or smaller than the half-value angle θ0H of the light-emitting device pass through the parallel central portion of prism 102 and are emitted with a substantially unchanged light intensity distribution. Meanwhile, light beams with the emission angle θ0 greater than θ0H pass through the inclined portion of prism 102 and are deflected toward the light axis. With this configuration, the relationship between the emission angle θ0 of the light beam emitted from the center of light-emitting device 101 and the emission angle θ1 which is the angle with respect to the principal light axis when the light beam is emitted from prism 102 is as shown in FIG. 24. As a result, the light beams emitted from the center of optical emitting module 100 substantially have the same radiation intensity at the emission angle θ1 of 0 to θ0H, as shown in FIG. 25.
Another known optical emitting module uses a lens (for example, Japanese Patent Unexamined Publication No. 2005-142447). FIG. 26 shows a known light emitting module described in Japanese Patent Unexamined Publication No. 2005-142447.
Referring to FIG. 26, optical emitting module 200 is configured such that light-emitting device 201 is incorporated in lens 203 made of resin or the like. The shape of refracting surface 203a of lens 203 is determined such that a light beam is refracted in accordance with the following relationship between an emission angle θ0 which is the angle of emergent light from the center of light-emitting device 201 with respect to the principal light axis and an emission angle θ1 which is the angle with respect to the principal light axis when the light beam is emitted from lens 203.cos θ1=1−(1−cosm+1θ0)(1−cos α)  (Expression 1)
Here, α denotes a desired radiation angle of light (the above-described transmission angle range: ±α), and m denotes the coefficient of a Lambertian distribution described below.
For example, when α=15° and m=1, as shown in FIG. 27, the emission angle θ0 and the emission angle θ1 have a relationship that, as the emission angle θ0 changes from 0° to 90°, the emission angle θ1 increases monotonically from 0° to 15°.
Expression 1 settles the emergent light distribution from light-emitting device 201 assuming that a change p(θ0) in the light radiation intensity with respect to the emission angle θ0 is a light radiation intensity distribution when a point light source with a Lambertian distribution expressed by the following expression is placed at the center of light-emitting device 201.p(θ0)=p0(1−cosm θ0)  (Expression 2)
Here, p0 denotes a light radiation intensity at θ0=0° (on the light axis), and m denotes the coefficient of a Lambertian distribution. Expression 1 simply expresses in a mathematical form that the light radiation intensity distribution of Expression 2 emitted in a range of −90°≦θ0≦90° (that is, a semispherical space at the front surface of the light-emitting device) is converted to a uniform light radiation intensity distribution in a space of −α≦θ1≦α outside lens 203. The expression is simply made in a mathematical form, thus if the light radiation intensity distribution is approximated to the Lambertian distribution on the minute light-emitting surface where light-emitting device 201 is close to the point light source, such that a substantially uniform radiation intensity distribution is obtained.
However, in the configuration of FIG. 23 using prism 102, the spread angle of light having passed through the prism is determined to be the half-value angle θ0H of light-emitting device 101. Thus, this cannot be applied to a case where the half-value angle θ0H and the transmission angle α are significantly different from each other. In the configuration of FIG. 23 using prism 102, since the size of the light-emitting device is not taken into consideration, when light-emitting device 101 does not have a minute light-emitting surface, the uniformity of the radiation intensity distribution may be significantly degraded.
In the configuration of FIG. 26 using lens 202, when light-emitting device 201 has a minute light-emitting surface, a uniform radiation intensity distribution can be formed in accordance with the transmission angle α, regardless of the half-value angle of light-emitting device 201. Meanwhile, since the size of the light-emitting device is not taken into consideration, when light-emitting device 201 does not have a minute light-emitting surface, the uniformity of the radiation intensity distribution may be significantly degraded.
The invention has been finalized in order to solve the problems inherent in the related art, and provides an optical emitting module that, even when the light-emitting surface of the light-emitting device is large, reduces the effect of the size of the light-emitting surface on the light radiation intensity distribution, obtaining a light radiation intensity distribution close to desired characteristics.